The present invention relates to A.C. plasma display panels and in particular to such panels for producing a multicolor display.
Plasma or gaseous discharge display and/or storage devices have certain desirable characteristics such as small size, thin flat display package, relatively low power requirements and inherent memory capability which render them particularly suitable for display. One example of such known gaseous discharge devices is disclosed in U.S. Pat. No. 3,559,190, "Gaseous Display and Memory Apparatus," patented Jan. 26, 1971 by Donald L. Bitzer et al and assigned to the University of Illinois. Such panels, designated A.C. plasma panels, may include an inner layer of physically isolated cells or alternatively comprise an open panel configuration of electrically isolated but not physically isolated gas cells. In the open panel configuration which represents the preferred embodiment of the present invention, a pair of glass plates having dielectrically coated conductor arrays formed thereon are sealed with the conductor arrays disposed in substantially orthogonal relationship. When appropriate drive signals are applied to selected conductors, the signals are capacitively coupled to the gas through the dielectric. When these signals exceed the breakdown voltage of the gwas, the gas discharges in the selected area, and the resulting charge particles, ions and electrons, are attracted to the wall having a potential opposite the polarity of the particle. This wall charge potential opposes the drive signal which produces and maintains the discharge, rapidly extinguishing the discharge and assisting the breakdown in the next alteration. Each discharge produces light emission from the selected cell or cells, and by operating at a relatively high frequency in the order of 30-50 kilocycles, a flicker-free display is provided. In general, the color of the emitted light is characteristic of or determined by the gas or mixture of gases employed in the gaseous discharge device. After the initial breakdown, the wall charge condition is maintained in selected cells by application of a lower potential control signal designated the sustain signal which, combined with the wall charge, causes the selected cells to be reignited and extinguished continuously at the applied frequency to maintain a continuous display.
In order to obtain a multicolor display using A.C. gas discharge display panels, the prior art has proposed using photoluminescent phosphors such as Zn.sub.2 SiO.sub.4 :Mn, YVO.sub.4 :Eu and CaWO.sub.4 :Pb incorporated into the panels. The phosphors are applied over the surface of the dielectric layer overlying the conductor arrays in donut or bar geometry and are excited by the ultra-violet radiation generated in the negative glow of a xenon, helium-xenon or helium-neon-xenon discharge.
Prior art multicolor A.C. plasma panels with open cell configuration which use photoluminescent phosphors include certain disadvantages such as optical cross talk between adjacent cells caused by line-of-sight excitation. Additionally, multiple reflection of ultraviolet radiation emanating from a cell in the "on" state seriously degrades on-off luminance. Another disadvantage of such prior art panels is that the luminous efficiency of the phosphor rapidly decreases due to degradation of the phosphor resulting from ion bombardment during the discharge.
The prior art has also taught certain methods for reducing optical cross talk and for protecting the phosphor from damage by the discharge in multicolor A.C. gas discharge display panels. One such method of reducing optical cross talk comprises the use of optical baffles to reduce line-of-sight excitation. Another method of reducing optical cross talk comprises using black ultraviolet-radiation-absorbing materials applied over the dielectric surface in selected areas surrounding the phosphors to reduce multiple reflection of ultraviolet radiation. However, suppression of optical cross talk achieved by these methods has not proven satisfactory.
In order to avoid degradation of the phosphor resulting from ion bombardment in a gaseous discharge device, a refractory material having a high binding energy and a high transmittance of ultraviolet radiation such as SiO.sub.2 or Al.sub.2 O.sub.3 is utilized to protect the phosphor. However, ion bombardment of SiO.sub.2 and Al.sub.2 O.sub.3 during A.C. operation substantially decreases the transmittance of ultraviolet radiation, resulting in a corresponding decrease in the luminance of the phosphor, thereby limiting the useful life of the device.